Combination cracking process for maximizing middle distillate production



Oct. 24, 1967 N J. PAIY'ERSON 3,349,023 1 COMBINATION CRA CKING PROCESS FOR MAXIMIZING MIDDLE DISTILLATE PRODUCTION Filed March 5, 1964 2 Sheets-Sheet 1 -----CASE B coNvRs|o-w-r.% GAS+ GASOLINE 00 FQEND POINT) FIG 1 033.4 H8383 Z, A1 sioncoad =10 013M INVENTOR NORMAN J. PA TERSON ATTOR r YS 3,349,023 IZING Oct. 24, 1967 N. J. PATERSON COMBINATION CRACKING PROCESS FOR MAXIM MIDDLE DISTILLATE PRODUCTION 2 Sheets-Sheet 2 Filed March 5, 1964 V m0; N 9 z I w R v S O m A N m w G 1 H 0 a do 63 Q N u. N d ql. O R 3 a 3 m w m M m m 8 NM% T O n S I 3 A Q A .n l m M m N O D 9 bx d n N w z w Y 3 0 a B N A N 3 R H d 3 VB wzoN w v 0z0 m0 .2 s Eimwxk E W LH 3 o w 0 3 a 3 9 2 3 m R 2 wzowzoN 1 o ozimomwm I 022920 1 09 0F J k 0 w -omo 2 I 0 2 & nn NM. 0 W G 2 N am 3 9 \N H NI K 0 02300 10 m :03 o a wzoN 2 Ar 025E3nmwo 1 0x2: A w 003 0 0 United States Patent Office 3,349,023 Patented Oct. 24, 1967 San Rafael, Calili, assignor to Company, a corporation of This invention relates to a process for the catalytic conversion of hydrocarbons to gasoline and middle distillate fractions. More particularly, the invention relates to a combination cracking process for the thermal and catalytic conversion of hydrocarbons, in order to maximize middle distillate production.

PRIOR ART The refiner is currently faced with the problem of how best to maximize the production of middle distillates due to the increased demand for jet 'fuels, stove and heating oils and diesel fuels. This problem exists in those areas where the demand for middle distillates fluctuates with the season and is particularly pertinent to those areas where middle distillate demand exceeds that of fuel oil and gasoline.

The refiner currently maximizes yields of middle distillates by undercutting gasoline and removing all middle distillate stocks from cracking plant feeds. Thus, he operates to the minimum flash point and the maximum pour point on his middle distillate production. At the same time, he conserves his middle distillate production by utilizing the lowest viscosity cutter stocks for blending with residuum to meet the maximum pour point and viscosity specifications on residual fuel oils. Further, he minimizes the use of middle distillates as cutter stock by thermal processing the long residuum in singleor two-coil cracking units or visbreaking units to effect maximum viscosity and pour point reduction on the residuum. Such units generally operate at low pressure and short time high temperature conditions to crack the heavy waxy gas oils to predominantly low pour point middle distillates with a minimum of gasoline production while obtaining some viscosity reduction on the asphaltic residuum.

These methods have improved the siutation, but have not significantly increased the ratio of middle distillate to gasoline production. It would be desirable if a method were available to produce higher ratios of middle distillates to gasoline than presently obtainable.

The increased usage of paratfinic crude oils, for example Middle East and north African crudes, has created a need for an economic processing scheme to produce low pour point residual fuel oils from such crude oils while maintaining a high ratio of middle distillates to gasoline and middle distillates to fuel oil. The increased availability of low sulfur north African crudes enables the refiner to meet the trend towards more stringent sulfur specifications for residual fuel oils. However, such crudes generally contain a higher percentage of straight-run gasoline components and the pour point of the residual fuel is to 50 F. higher than corresponding residual fuel from most Middle East crudes. It would be desirable if a method were available to produce a salable, low pour residual fuel oil while maintaining a high ratio of middle distillates to gasoline and to fuel oil from such paraffinic crude oils.

OBJECTS In view of the foregoing, it is an object of the present invention to provide a process wherein the ratio of middle distillates to gasoline production is increased to an extent heretofore not possible.

It is a further object of the present invention to provide a process that is more economical and flexible in meeting product demands and qualities than existing processes.

DRAWINGS The invention will be best understood, and further objects and advantages thereof will be apparent from the following description, when read in conjunction with the accompanying drawings, in which:

FIGURE 1 is a graphical representation of the improvement in middle distillate yields obtained by thermal processing in accordance with the process of the present invention;

FIGURE 2 is a simplified diagrammatic process flow diagram of a refinery incorporating the process of the present invention.

STATEMENT OF THE INVENTION The present invention provides an improved process wherein a hydrocarbon feed boiling above 550 F., other than a hydrocrackate boiling above about 550 F., is thermally cracked in a thermal cracking zone to produce synthetic materials, including gasoline and middle distillates, and wherein as the operating temperature is constantly raised, the yields of gasoline and middle distillates increase up to a critical conversion point above which gasoline yield continues to increase in a substantially constant rate but middle distillate yield begins to decrease. In accordance with the present invention the critical conversion point is raised by passing to said thermal cracking zone, along with said hydrocarbon feed, a hydrocrackate boiling above about 550 F. in an amount of at least one volume per ten volumes of said hydrocarbon feed.

Further, in accordance with the present invention, there is provided, in a process wherein a hydrocarbon feed boiling above 550 F. is thermally cracked in a thermal cracking zone operating at conditions under which 10 to 30 wt. percent of said hydrocarbon feed would be converted to synthetic products boiling below 300 F in the absence of the hydrocarckate, to produce synthetic materials, including gasoline and middle distillates, and wherein as the op erating temperature is constantly raised the yields of gasoline and middle distillates increase up to a critical conversion point above which gasoline yield continues to increase at a substantally constant rate, but middle distillate yield begins to decrease, a method for raising said critical conversion point, which comprises passing to said thermal cracking zone, along with said hydrocarbon feed, a hydrocrackate boiling above about 55 0 F. in an amount of at least one volume per 10 volumes of said hydrocarbon feed. At least 30 volume percent of the unconverted materials from the thermal cracking zone are converted in a hydrocracking zone to products boiling below about 700 F. and the effluent from said hydrocracking zone, boiling above the end point of said products, at least above 550 F is passed to said thermal cracking zone.

As used herein, the unconverted bottoms from the hydrocracking zone boils entirely above 550 F. though it may contain a slight amount of lower boiling materials resulting from the incomplete separation of 550 F. end point middle distillates.

By means of the process of the present invention, it is possible to markedly increase the yield of middle distillates per barrel of crude oil and, at the same time, increase the ratio of middle distillates to fuel oil while maintaining constant gasoline production. In addition, residual fuel oils having lower sulfur contents and lower pour points are produced.

PROCESS UNITS AND OPERATING CONDITIONS, GENERAL The process of the present invention includes the combination of a thermal cracking zone and a hydrocracking zone. The latter may be either a oneor two-stage system with the necessary source of hydrogen. Suitable operating conditions for these two zones are described below.

THERMAL CRACKING ZONE The thermal cracking zone fresh feed stock may be any of the conventional types such as, for example, a bottoms or residuum fraction obtained from a crude distillation column which boils above about 550 F.

The thermal cracking unit may be a singleor multi-coil unit which operates under conditions of feed rate, temperature, and recycle rate such that '10 to 30 wt. percent of the fresh thermal cracking feed is converted to synthetic materials boiling below 300 F. For example, the thermal cracking unit may comprise a single-coil furnace which converts the residum feed at a conversion per pass of 4 to 7 volume percent based on the production of 300 F. end point gasoline. Conversion per pass used herein is the ratio of the C 300 F. end point gasoline production in.

barrels per day (b.pd.) divided by the b.p.d. of total furnace feed (fresh feed plus recycle) and multiplied by 100. Thus, if the C -300 F. gasoline production is 1500 b.p.d., the fresh feed to the cracking unit is 10,000 b.p.d. and the recycle is 20,000 b.p.d., the conversion per pass is 5.0 volume percent. Alternately, the thermal cracker feed may be processed in a two-coil unit wherein the primary or black oil coil operates at low pressuresof 50 to 150 p.s.i.g. measured at the coil outlet and a conversion per pass of 1 to 3 volume percent. The secondary or clean oil coil operates on gas oil recycle at pressures of 150-250 p.s.i.g. measured ume percent. The effluent from the singleor the multi-coil furnace is passed, after pressure reduction and quenching to a flash separator for the separation of the cracked tar or residuum, and a wide-cut distillate stock. The flash separator operates at a pressure of 25 to 100 p.s.i.g. and a bottoms temperature of 560 to 825 F. The cracked tar from the flash separator is passed, without cooling, to a vacuum stripper where a heavy waxy vacuum gas oil recycle boiling in the range of about 700 to 1000 F. is separated from the cracked tar.

If the thermal cracking zone feed is a vacuum residuum from a two-stage crude unit, and primary coil of the thermal cracking unit operates under visbreaking conditions of 50 to 150 p.s.i.g. pressure at the coil outlet and a conversion per pass of 1 to 3 volume percent.

The coiloutlet temperature range is 750 to 950 F. for the primary or black oil coil and 825 to 1025 F. for the secondary or clean oil coil.

To improve operation of both singleand ,multi-coil thermal cracking units, 1 to 2 wt. percent of water may be injected into the coil inlet tubes of the furnaces to promote short time-high temperature cracking, to reduce coil coking and to give a fuel oil product of higher stability and compatibility.

HYDROCRACKING ZONE The feed to the hydrocracking zone may be any suitable gas or residual oil such as straight-run gas oil, deasphalted residual oil, thermal crackergas oil or catalytic cycle oil. In the preferred embodiment of the present invention, the feed to the hydrocracking zone is a thermal gas oil boiling above about 550 F.

Depending on the particular hydrocracking catalyst used, the hydrocracking zone feed may contain relatively large quantities of nitrogen. There is no nitrogen limit on the hydrocracking zone bottoms which are passed to the thermal cracking zone although, under the severe hydrogenation conditions employed, the nitrogen is reduced to very low levels.

It is well known that nitrogen has an extremely deleterious effect upon the hydrocracking performance of acidic hydrocracking catalysts. It is also well known that the nitrogen content of the feed to the hydrocracking zone containing such a catalyst should be kept below about 50 at the coil outlet and a conversion per pass of 5 to 12 volppm, preferably below about 10 p.p.m. and, still more preferably, below about 2 ppm; of nitrogen. A low level of nitrogen in the feed permits the hydrocracking reaction over an acidic-type catalyst to be conducted at lower tem-. peratures than feeds containing larger amounts of nitrogen compounds.

In cases in which the hydrocracking zone contains an acidic-type catalyst and the feeds are not inherently low in nitrogen, the feed may be hydrofined by conventional catalysts make excellent hydrofining catalysts. Thus, they can be used in a separate hydrofining stage for the hydro= cracking stage containing a non-acidic or weakly-acidic catalyst where the-feed contains more than 1000 p.p.m. nitrogen or containing an acid catalyst.

The usual hydrocracking zone operating conditions comprise from about 2000 to 30,000 s.c.f. hydrogen/bbl. of total feed and, preferably, from about 2000 to 15,000, at an LHSVof from about 0.2 to 15 and, preferably, from about:0.4 to about 3.0, at a pressure of'at least 1000 p.s.i.g. and, preferably, from about 1000 to 3000 p.s.i.g. and .a temperature in the range of from about 400 to 950 F. The preferred initialon-stream temperature is from about 500 to 650 F., with progressive increase to about 750 to 950 F., so as to maintain catalyst activity at a controlled level.

The catalyst used in the hydrocracking zone is one wherein a material having a hydrogenating-dehydrogenating activity is deposited or otherwise combined with a cracking component. The cracking component may cornprise any one or more non-acidic, weakly-acidic or strongly-acidic materials such as silica, alumina, bauxite,

ing-dehydrogenating component may be present and' favorable results may be obtained with catalysts containing composites of two or more of the oxides of molybdenum, cobalt, chromium, tungsten and nickel, and with mixtures of said oxides with fluorine. The amount of the hydrogenating-dehydrogenating component can be varied within wide limits from about 0.5 to 30% based on the weight of the entire catalyst.

The proper selection of operating conditions and catalysts should be correlated with the particular type of hydrocracking feed stock used, in order to convert at least 30% of the fresh feed to products boiling below the initial boiling point of the feed. The minimum hydrogen consumption in order to obtain this conversion is 750 s.c.f./bbl. feed. In general, an acidic catalystsuch as nickel sulfide and/or cobalt sulfide combined with such acidic materials as a conventional cracking catalyst containing silica-alumina, silica-alumina-zirconia, acid treated clays and the like, is used for hydrocracking the hydrofined light straight-run and catalytic cycle oils; and

a non-acidic catalyst, such as a combination of cobalt sulfide and/or nickel sulfide with molybdenum sulfide combined with such non-acidic materials as alumina (preferred), bauxite, silica and zirconia or a weaklyacidic catalyst such as a combination of nickel sulfide and/or. molybdenum sulfide with tungsten sufide, combined with such weakly-acidic materials as silica-magnesia, are used for hydrocracking heavy straight-run and thermal gas oils and deasphalted residual oils.

DESCRIPTION In order to further illustrate the practice and advantages of the present invention, reference is made to FIGURE 1 which shows a graphic illustration of the yields obtainable by the process of the present invention, compared to the yields obtainable by the conventional thermal cracking of the same feed stocks. The pilot plant data which forms the basis for the solid lines Was obtained by thermal cracking a long residuum, boiling above 675 F., from an atmospheric distillation of a 37.1 API north Arican Libyan crude oil in a single coil pilot plant thermal cracking unit and recycling the vacuum gas oil boiling between 675 and 1000 F. The thermal cracker was operated at a coil outlet temperature of 880 F., a pressure of 2.50 p.s.i.g., at a fresh feed rate of 1.20 gal/hr. and at a total feed (fresh recycle) rate of 4.65 gal/hr.

The solid lines show the liquid volume percent yields from the fresh feed to a thermal cracking zone, which include a C 300 F. gasoline, a 300 to 675 F. middle distillate and a cracked tar. These results indicate the limiting yields obtainable by conventional thermal cracking and will be referred to herein as Case A.

The dotted lines show the liquid volume percent yields of the same product-s from the fresh feed to the thermal cracker operating at the same conditions of temperature and pressure as Case A, but with 1.3 volumes of hydrocracker bottoms boiling above 675 F. added per volumes of residuum. These yields were obtained from correlations based on pilot plant data wherein a thermal recycle gas oil similarly obtained was hydrocracked in a once-through single stage hydrocracker over nickel sulfide and tungsten sulfide on silica-alumina at 780 F., 2000 p.s.i.g., and a hydrogen consumption of 1800 s.c.f./ bbl. of feed and wherein the unconverted bottoms from the hydrocracker was combined with the residuum to the thermal cracker.

The dotted lines indicate the increased yields obtainable by operating in accordance with the present invention, i.e., adding at least one volume of a substantially hydrogenated bottoms from a hydrocracking zone, operating on a once-through basis, to 10 volumes of fresh thermal cracking zone feed, and will be referred to herein as Case B.

It can be seen from FIGURE 1 that, in both cases, as the thermal cracking zone conversion increases, the yield of C 300 F. thermal cracked gasoline increases at a constant rate. The yield of cracked tar, in both cases, decreases with increased conversion until approximately 50% conversion is reached, at which time the yield of cracked tar remains approximately constant with increased conversion. However, in Case B the yield of cracked tar at any conversion is less than in Case A. It will also be observed that, as conversion increases, the yield of 300 to 675 F. middle distillates in both cases increases up to a critical conversion point above which gasoline yield continues to increase at substantially constant rate but middle distillate yield begins to decrease. This critical conversion point is increased from approximately 26, in Case A, to approximately 31, in Case B. The net result, by means of the process of the present invention, is to markedly increase the volumetric yield of middle distillates over the yield obtained by conventional thermal cracking at constant gasoline yield. It must be emphasized that the product yields indicated in FIGURE 1 are obtained from the thermal cracker only and do not include those obtained from the hydrocracker.

In addition, FIGURE 1 indicates that, although the ratio of middle distillates to gasoline constantly decreases with increased conversion, the ratio at any given conversion point is greater for Case B than Case A. The increase in middle distillates that is obtained in Case B over that obtained in Case A is at the expense of cracked tar and gas.

In a conventional thermal cracker, as conversion increases, the olefinic content of all the products increase, including the gas oil recycle. This increase in olefinic content with an increase in conversion, has a tendency to increase the rate of condensation and polymerization, and also the rate of decomposition to gasoline and gas. Although not intended to be limited to any one theory, or explanation for the results obtained in the process of the present invention, it is believed that the addition of a substantially hydrogenated bottoms tends to slow down the rates of polymerization and decomposition to gasoline and gas. Thus, this makes more material available for conversion into middle distillates.

Referring now to FIGURE 2, there is shown an exemplary over-all process flow diagram suitable for carrying out one or more preferred embodiments of the present invention.

A crude oil charge stock, which may be a single crude oil or a mixture of crude oils, is heated and passed through line 1 to crude column 2. The feed stock in crude column 2 is distilled at substantially atmospheric pressure into a wide-cut distillate overhead, boiling below about 700 F. The overhead is passed through line 3, combined with hydrogen through line 4 and passed into hydrodesulfurizing zone 5. Zone 5 contains a conventional desulfurizing catalyst (such as cobalt-molybdenum on alumina) which saturates the olefins and removes sulfur and a portion of the nitrogen from said overhead. The eflluent from zone 5 is condensed in contact with injected condensate water, cooled and passed to a separating drum wherein the condensate water containing the dissolved ammonia and hydrogen sulfide is drawn off and the recycle hydrogen stream is flashed oiT to return to the hydrogen inlet of zone 5, not shown. The efiluent from zone 5 is then passed through line 6 to distillation column 7 for the recovery of products.

The residium from crude column 2 is removed by line 10 and passed to a thermal cracking zone 11 operating at the above conditions such that 10 to 30 wt. percent of said residium is converted to synthetic products boiling below 300 F. By way of specific example, the residuum is thermally cracked in a single recycle coil at 250 p.s.i.g., a 5.5 volume percent conversion per pass and 3.5 :1 combined feed ratio. (Ratio of fresh feed recycle to fresh feed.) The eflluent from the thermal cracking zone 11 is quenched to a transfer line temperature of about 775 F. and then pressure reduced and passed through line 12 to flash separator 13 operating at 15 p.s.i.g. In zone 13, the eflluent is flashed to give a wide-cut distillate fraction boiling below about 700 F. This fraction is passed through line 14, combined with the overhead through line 3, and passed to hydrodesulfurizing, zone 5. The flashed residuum from zone 13 passes through line 15 to vacuum stripper 16 operating at 10 mm. Hg absolute where it is seperated into a wide-cut waxy vacuum gas oil of which all or a portion thereof may be recycled through line 17 to the inlet to thermal cracking zone 11. The cracked tar bottoms from the vacuum stripper may be passed through line 18 to product storage as a bunker fuel oil component or to subsequent processing, such as in a solvent deasphalting unit. A portion of the wide-cut vacuum gas oil may be combined with a deasphalted oil, catalytic cycle oil, straight-run gas oil or other suitable gas oil which is passed through line 20, and is passed through line 19 with hydrogen through line 21 to hydrocracking zone 22. Hydrocracking zone 22 operates with a suitable hydrocracking catalyst under conditions as discussed above, in order to convert at least 30% of the feed to products boiling below the initial boiling point of the feed. By way of a specificexample, thevacuum gas oil is hydrocracked in a once-through single-stage hydrocracker over nickel sulfide (6 /2 wt. percent Ni) and molybdenum sulfide oils to produce a maximum yield of middle distillates using a conventional processing arrangement of two-coil thermal cracking with vacuum flashing the cracked tar, compared with a material balance. obtained by processing the same feeds using a processing arrangement in accordance with one embodiment of the present invention, i.e. two-coil thermal cracking with vacuum flashing the cracked tar and single-stage hydrocracking a part of the vacuum gas oil, as described in connection with FIGURE 1. The primary coil of the thermal cracker operates once through at 750 to 950 and 50 to 150 p.s.i.g. at the coil outlet. The secondary coil operates at 825 to 1025 and 250 to 650 p.s.i.g. at the coil outlet.

EXAMPLE 1 [50,000 b.p.s.d. Refinery Processing 31.7 API Kuwait and 371 API Libyan Crude Oils] Conventional Case A Present Invention Case 13 Thermal cracking with Thermal cracking with vacuum flashing the cracked vacuum flashing the cracked Processing Arrangement tar tar and hydrocrncking a part of vacuum gas oil Kuwait Crude Libyan Crude Kuwait Crude Libyan Crude Refinery Input, b.p.s.d 50, 000 50, 000 50,000 50, 000 Refinery Products:

C -EFO 1 2, 800 3,000 2,800 3, 000 C3-C4 LPG, b.p.s.d 2, 000 2, 000 2,000 2, 000 Motor gasoline, 10 lb. RVP 95 b.p.s.d 5 11, 0 10, 100 11,350 Middle distillate (300-675 F.), 0 F. pour, b.p.s.d 18, 490 25, 790 23, 700 29, 550 No. residual fuel oil, b.p.s.d 17,2 0 8,300 12,150 4, 900 500 S.S.U./l F. vise. pour point, F- +30 +35 Sulfur, wt. percent 3.0 0. 7 3.0 0. 7 Middle Distillate/Fuel Oil 1.07 3. 10 1.95 6, 03 Middle Distillate/Gasoline 1. 94 2. 34 2. 35 2. 60 Processing Rates, b.p.s.d.:

Two-coil thermal cracker:

Residuum feed 23, 500 15, 500 Hydrocracker bottoms 2, 700 1, 860 Single-stage hydrocracker, once-through 5,000 4, 000 Catalytic Reformer 8,600 10, 600

1 EFO (Equivalent Fuel Oil) is b.p.s.d. of 10 API bunker fuel that would have equivalent heating value in B.t.u., assuming that one barrel of sand fuel oil has a heating value of 6.3 M B .t.u.

2 Reid Vapor Pressure.

removed to line 25 for further processing. Alternatively,

the hydrocracker feed rate may be controlled so that all of the hydrocracker bottoms is passed to thermal cracking zone 11.

The total products from the preferred unitary distillation scheme are removed as side cuts from distillation column 7. The propane and lighter boiling materials are removed from the system by line 29. Mixed C -C s are removed to product storage by line 30 asLPG. The

light straight-runand hydrocracked (C l80F.) gasoline is removed through line 31. The heavy straight-run and hydrocracked (180 to 300 F.) gasoline is passed through line 32 to catalytic reforming zone 33. Catalytic reforming zone 33 may operate under conventional catalytic reforming conditions with a conventional. reforming catalyst such as platinum on aluminum. The reformate from reforming zone 33is passed through line 34, combined with the C 180 F. gasoline, and removed to product storage through line '35 as a high octane gasoline. The middle distillates are removed to product storage through line 36. Alternatively, the middle distillate can be split into a 300 to 480 F. kerosene or jet fuel product and a 480 to 675 F. diesel fuel product.

EXAMPLES Example 1 is a material balance obtained by processing 50,000 b.p.s.d. of north African and Middle East crude EXAMPLE 2 [50,000 b.p.s.d. Refinery Processing Blend of Kuwait and Libyan Crudesl Present Invention Conventional Thermal Thermal cracking with Processing Arrangement cracking with vacuum vacuum stripping the stripping the cracked tar cracked tar and hydrocracking a part of vacuum gas oil Refinery Input, b.p.s.d.:

Kuwait Crude. 25,000 30,000 Libyan Crude 25, 000 20,000 Refinery Products:

Motor gasoline, F-1+0.5 ml.

TEL/gr t, b.p.s.d 10, 270 10,725 Middle distillate, 0 F. pour point,

b.p.s,(1 22,140 26, 625 No. 5 fuel oil, 500 S.S.U./ F.,

b. .S.d 12, 775 8, 525 Pour point, F. +40 +30 Sulfur, wt. percent. 2.0 2.0 Middle Distillate/Fuel Oil. 1. 72 3.10 Middle Distillate/Gasoline 2. 15 2. 50 Processing Rates, b.p.s.d.:

Two-coil thermal cracker:

Residuum feed 19, 500 19, 500 Hydroeraeker bottoms 2, 375 Single-stage hydrocracker 4, 500 Catalytic reformer 9, 9, 000

These examples show that, compared with the conventional refining arrangement, the process of the present invention results in a large increase in the middle distillate to fuel oil ratio and the middle distillate to gasoline ratio. Example 2 shows, by blending the two cnides, the refinery is able to maintain these high ratios and meet the 2 weight percent sulfur specification on the low pour fuel oil.

Although only specific embodiments of operation of the present invention have been described, numerous variations could be made in those embodiments without departing from the spirit of the invention and all such variations that fall within the scope of the appended claims are intended to be embraced thereby.

What is claimed is:

1. In a process wherein a hydrocarbon residuum feed boiling above about 550 F., other than a hydrocracked fraction as described hereinafter, is thermally cracked in a thermal cracking zone to produce synthetic materials including gasoline and middle distillate, and the efiluent thereof is separated into fractions including at least a gasoline fraction, a middle distillate fraction having an end boiling point of at least 550 F., a distillate gas oil fraction boiling above the end boiling point of said middle distillate fraction, and a cracked tar fraction boiling above the end boiling point of said gas oil fraction, and wherein conversion of said feed to said synthetic products increases as operating temperature is raised up to a critical conversion point above which gasoline yield continues to increase as operating temperature is further raised but middle distillate yield begins to decrease;

the improvement for raising said critical conversion point to increase the yield of middle distillate and decrease the yield of cracked tar without substantially changing gasoline yield, said yields being with reference to said hydrocarbon feed, which comprises:

catalytically hydrocracking a hydrocarbon oil boiling above about 550 F., separating the oil efiluent of the hydrocracking into fractions including middle distillate and a hydrocracked fraction boiling above the end boiling point of said middle distillate fraction separated from the efiuent of said thermal cracking zone, and passing at least a portion of said hydrocracked fraction to said thermal cracking zone in an amount of from one to ten volumes per ten volumes of said hydrocarbon feed.

2. Process in accordance with claim 1 wherein said thermal cracking zone is operated at conditions under which from 10 to 30 weight percent of said hydrocarbon feed would be converted to synthetic products boiling below 300 F. in the absence of said hydrocracked fraction.

3. Process in accordance with claim 1 wherein said hydrocarbon oil Which is catalytically hydrocracked comprises a portion of said distillate gas oil fraction separated from the eflluent of said thermal cracking zone, another portion of said gas oil fraction being recycled to said thermal cracking zone.

4. Process in accordance with claim 3 wherein at least 30 volume percent of said distillate gas oil fraction is catalytically hydrocracked to products boiling below about 700 F.

References Cited UNITED STATES PATENTS 11/1961 Hansford et al. 208-6 8 9/1964 Tulleners 208111 

1. IN A PROCESS WHEREIN A HYDROCARBON RESIDUUM FEED BOILING ABOVE ABOUT 550*F., OTHER THAN A HYDROCRACKED FRACTION AS DESCRIBED HEREINAFTER, IS THERMALLY CRACKED IN A THERMAL CRACKING ZONE TO PRODUCE SYNTHETIC MATERIALS INCLUDING GASOLINE AND MIDDLE DISTILLATE, AND THE EFFLUENT THEREOF IS SEPARATED INTO FRACTIONS INCLUDING AT LEAST AS GASOLINE FRACTION, A MIDDLE DISTILLATE FRACTION HAVING A END BOILING POINT OF AT LEAT 550*F., A DISTILLATE GAS OIL FRACTION BOILING ABOVE THE END BOILING POINT OF SAID MIDDLE DISTILLATE FRACTION, AND A CRACKED TAR FRACTION BOILING ABOVE THE END BOILING POINT OF SAID GAS OIL FRACTION, AND WHEREIN CONVERSION OF SAID FEED TO SAID SYNTHETIC PRODUCTS INCREASES AS OPERATING TEMPERATURE IS RAISED UP TO A CRITICAL CONVERSATION POINT ABOVE WHICH GASOLINE KYIELD CONTINUES TO INCREASE AS OPERATING TEMPERATURE IS FURTHER RAISED BUT MIDDLE DISTILLATE YIELD BEINGS TO DECREASE; 